PROJECT SUMMARY/ABSTRACT Clonal hematopoiesis of indeterminant potential (CHIP) is an aging-associated condition that confers increased risk of progression to hematologic disorder and a decreased overall survival rate. The incidence of CHIP increases substantially with advancing age, present in 10-15% of individuals aged 70 years or older. CHIP is a significant health concern as the proportion of individuals ? 65 years old in the United States is expected to increase from 15% of the population in 2014 to 24% by the year 2060. CHIP is caused by somatic mutations that confer a selective advantage to hematopoietic stem cells (HSCs) and their progeny, which can be easily detected using next-generation sequencing. However, there are currently no methods to identify individuals in which CHIP will progress to hematologic disorder, and no therapies to prevent this progression. The long-term goal of this research is to develop novel strategies to extend healthy hematopoietic function during aging and prevent aging-associated hematologic disorders. The overall objective of this proposal is to determine the aging-associated cellular and molecular alterations that promote the expansion of CHIP-mutant clones and their progression to hematologic disorder. The rationale is that the underlying mechanisms will be prime, modifiable targets for detection and mitigation of high-risk CHIP. Preliminary data describe a technically innovative mouse model in which the timing of CHIP development and progression can be strictly controlled. This is the only model to date that recapitulates in vivo CHIP progression as it occurs in humans. Using this model, the aged bone marrow (BM) microenvironment was shown to accelerate CHIP expansion and progression, and correlated with an age-related increase in the concentration of pro-inflammatory cytokines in the BM microenvironment. These data support the central hypothesis that alterations in the aged BM microenvironment increase the selective advantage of CHIP-mutant HSCs and their progeny causing CHIP expansion and progression. This project will use cellular and molecular biological approaches in aged mice to achieve the following specific aims: AIM 1. Discover pathogenic somatic mutation(s) selected for by an aged BM microenvironment during CHIP expansion and progression; AIM 2. Delineate the mechanisms by which the aged BM microenvironment accelerates CHIP expansion and progression; and AIM 3. Determine the extent to which chronic inflammation causes CHIP expansion and progression. The proposed research is conceptually innovative because it is the first to define the important role of the aged BM microenvironment in causing CHIP expansion and progression. This study is significant because it will define the cellular and molecular mechanisms that underlie progression of CHIP to hematologic disorder and provide a fundamental basis for the development of biomarkers to predict high-risk CHIP and novel prophylactic therapies to mitigate CHIP progression.